The discovery of 13s X-ray pulsations from the hydrogen depleted subdwarf O6 star binary HD49798
G. L. Israel, L. Stella, L. Angelini, N. E. White, T. R. Kallman, P. Giommi, A. Treves
Abstract
We discovered strong 13s X-ray pulsations in the public Rosat PSPC light curve of HD49798, a 1.55d single-component spectroscopic binary containing a hydrogen depleted subdwarf O6 star. We find no evidence for period changes during the about 4hr Rosat pointing. The source X-ray spectrum is extremely soft, with an unabsorbed 0.1-2keV luminosity of a few times 1032 erg s-1 (distance of 650pc). A higher luminosity might be hidden in the EUV. Our results imply that the unseen companion is an accreting degenerate star, a white dwarf or, more likely, a neutron star. In either case HD49798 corresponds to a previously unobserved evolutionary stage of a massive binary system, after common envelope and spiral-in.
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